Conventional “anti-friction” rolling element bearings have been available for many years made of numerous materials for numerous different applications. Their primary benefit, low starting friction, has earned them a permanent place in the phalanx of standard mechanical elements used in most products having moving parts, but they are notorious for a host of long-standing intractable problems. Steel, aluminum, brass, bronze, Monel, silicon nitride, plastics and ceramics are known materials used for their desirable properties in bearings, but all suffer from deficiencies that make their use in bearings less than ideal.
One of the most serious problems of conventional rolling element bearings is corrosion. Even though such bearings are normally used with a lubricant, which provides some degree of protection from corrosive agents, the lubricant can itself become contaminated with those corrosive agents. Even “sealed” bearings are designed to vent at low atmospheric pressure to avoid blowing out the seal, and then corrosive agents can be drawn into the bearing case when the atmospheric pressure returns to normal. Also, corrosive particles can become embedded in the surface of bearing elements and races which are softer than the particles, and the embedded corrosive particles cause rapid corrosive pitting of the bearing elements and races, resulting in early failure of the bearing.
Corrosion resistant rolling bearing elements and races have been developed, but all suffer from deficiencies. Most seriously, the corrosion resistant bearings are soft and have low strength, so even if the corrosion resistance they offer does actually allow the bearing to operate in a corrosive environment without corrosion, they have a limited life because of accelerated wear and fatigue sensitivity. Moreover, few of the so-called corrosion resistant bearings are really corrosion-proof. Corrosion pitting in a rolling bearing element or race results in early destruction of the bearing. Replacing corroded bearings reduces the availability of the equipment in which the bearings are used and greatly increases operating costs for replacement, maintenance and down time.
Bearings in equipment used in the food processing industry require particular scrutiny because of the danger of contamination of the food with metal particles, lubricant and the likelihood of failure because of corrosion resulting from use of chloride cleaners on the equipment. As a result, bearings used in food processing machinery are often made of soft “corrosion resistant” materials, but wear particles from such bearings are a constant source of concern and require continual costly maintenance and down-time to replace the short lived bearings.
Modern equipment often requires non-magnetic bearings. This requirement has been met in the past by non-magnetic materials such as brass and some stainless steels. However, these materials lack the strength of conventional steel bearings, so they must be designed over-sized to provide the needed load-carrying capacity. Moreover, most non-magnetic bearing materials are susceptible to corrosion and must be replaced frequently to avoid catastrophic failure from corrosion, resulting in costly and disruptive down-time for the equipment in which they are installed. Non-magnetic stainless steel is a fair solution to the problem, but the magnetic properties of stainless steel are not always predictable and can vary from batch to batch.
Ceramic rolling elements were thought to solve the magnetic and corrosion problems, and in fact are less susceptible to corrosion than the “corrosion resistant” metallic bearings. However, they have introduced their own unique set of deficiencies that make their use limited to a small segment of the market. They are very costly, so they are used only where there is no other alternative. They are fracture sensitive and fracture of a ceramic ball produces instantaneous failure of the bearing, so the equipment in which they are used must be designed to isolate the bearing from impacts. They are made of material that is very abrasive and can produce accelerated wear of the races and each other if allowed to rotate in contact with each other. They produce wear products and damage particles that cannot be detected by conventional equipment, so they are unsuitable for food machinery. They can be used in ceramic races, but ceramic races are also expensive and fracture sensitive. When ceramic balls are used in metallic races to reduce the fracture sensitivity of the bearing, the corrosion and magnetic problems solved by the ceramic rolling elements do nothing to solve the same problems with the races.
Nitinol is a nickel-titanium intermetallic compound invented at th Naval Ordinance Laboratory in the early 1960's. It is a material with useful properties, but manufacturers who have worked with it have had little success in making Nitinol parts and semi-finished forms, and have never attempted to make bearings of Nitinol. Because Nitinol is so extremely difficult to form and machine, workers in the metal products arts usually abandoned the effort to make products out of anything except drawn wire because the time and costs involved did not warrant the paltry results they were able to obtain.
Nitinol, particularly Type 60 Nitinol (60% Nickel and 40% Titanium by weight), has many properties that are unrecognized as of potential value in bearings. It can be polished to an extremely smooth finish, less than 1 microinch rms. It is naturally hard and can be heat treated to a hardness on the order of 62Rc or higher. It can be processed to have a very hard integral ceramic surface that can itself be polished to an even smoother surface than the parent metal. It is non-magnetic, immune to corrosion from most common corrosive agents, and has high yield strength and toughness, even at elevated temperatures. It is 26% lower density than steel for high revolution rate applications and for weight sensitive applications such as aircraft, satellites and spacecraft. However, there has hitherto been no attempt to make bearings out to Nitinol because it is so difficult to work, because it was known to be brittle, and because there has been no known method to make rolling elements and races out of Nitinol.